Microstructure and mechanical properties of bobbin tool friction stir welded ZK60 magnesium alloy

Bobbin tool friction stir welding (BTFSW) technique was applied to a Mg–Zn–Zr alloy ZK60. The results indicated that the plates were joined successfully with no welding defect forming at the investigated processing parameters. BTFSW resulted in significant grain refinement and dissolution of the Mg₄Zn₇ precipitates in the stir zone (SZ). Extensive softening was observed in the SZ because the strengthening resulted from the grain refinement could not compensate the strength loss caused by the precipitates dissolution. The ultimate tensile strength of the joint reached 80.3–84.4% of that of the ZK60 parent metal. All the joints consistently fractured in the SZ adjacent to the thermo-mechanically affected zone (TMAZ) on the advancing side (AS). The extensive softening in the SZ makes it more susceptible to plastic deformation during transverse uniaxial tension, which is the precondition for the failing in the SZ-side. Within the SZ, the profuse activation of {10–12} extension twinning in the SZ-side on the AS (with <0001> crystal direction parallel to transverse direction) and basal slip in the adjacent region (with <0001> crystal direction at ~45° to transverse direction) caused deformation incompatibility. Additionally, the abrupt change of microstructure from the TMAZ to the SZ aggravated the deformation incompatibility, leading the fracture to initiating in the SZ-side on the AS.